1. Field of the Invention
This invention relates to a hydrogen-absorbing alloy used for a negative electrode in an alkaline storage battery.
2. Description of the Prior Art
Alkaline storage batteries, such as Ni-Cd batteries, or lead acid batteries, are known. Currently, lighter and larger capacity batteries with high energy density are needed. Therefore, metal hydride storage batteries, which utilize a hydrogen-absorbing alloy to reversibly absorb and desorb hydrogen as a negative electrode, are attractive.
The hydrogen-absorbing alloy for the negative electrode should be able to absorb and desorb reversibly at room temperature. An alloy satisfying this requirement is disclosed in the laid-open Japanese patent publication No. 60-89066, wherein a hydrogen-absorbing alloy of an Mm-Ni type is described. The "Mm" a "Misch Metal" is a mixture of rare earth elements, such as lanthanum (La), cerium (Ce), neodymium (Nd) and praseodymium (Pr). These alloys have been used in practice.
Electrochemical characteristics of the hydrogen-absorbing alloy electrode and the charge characteristics of an alkaline storage battery, that utilizes the hydrogen-absorbing alloy as a negative electrode, depend on various characteristics of the alloy used. Therefore, it is important to research and develop proper hydrogen-absorbing alloys for the electrode.
For example, in a LaNi.sub.5 alloy, an MmNi.sub.5 alloy, partial replacements of lanthanum (La), "Mm" or nickel (Ni) by other elements improves the discharge capacity, temperature dependency and charge-discharge cycle life characteristics. Accordingly, such replacements are often researched and tested.
When nickel (Ni) is partially replaced by cobalt (Co), copper (Cu) or another appropriate replacement, a change of volume in the alloy is controlled during charge-discharge cycles. As a result of this control, the alloy is prevented from flaking off of the electrode, and the charge-discharge cycle life of the battery is improved. Similarly, a partial replacement by manganese (Mn), aluminum (Al) or the like decreases the equilibrium-pressure of the hydrogen-absorbing alloy. Consequently, the amount of absorbed hydrogen is increased, as is the discharge capacity of the battery.
Another method of decreasing the equilibrium-pressure in the hydrogen-absorbing alloy, with a composition MmBx, involves the reduction of the stoichiometric ratio "X" by an amount that element "B" is reduced. For example, the laid-open Japanese patent publication, No. 60-89066, describes a hydrogen-absorbing alloy, ABmCn, with a value of "m+n" in the range 4.8 to 5.4. Further, the laid-open Japanese patent publication No. 2-277737 describes a hydrogen-absorbing alloy represented by formula ANiaCobMnc, wherein the value of "a+b+c" ranges from 3.85 to 4.78. These methods increase the amount of the absorbed hydrogen in the hydrogen-absorbing alloy.
In manufacturing the hydrogen-absorbing alloy, each component of the alloy is measured and mixed in a fixed ratio, melted in an arc furnace with an inert argon atmosphere, and cooled in order to obtain hydrogen-absorbing alloy bulk. The alloys are then held in a vacuum furnace and heat treated at a high temperature (approximately 1000.degree. C.) for a fixed period of time. This "annealing" heat treatment causes homogenization of the hydrogen-absorbing alloys. For instance, the laid-open Japanese patent publication No. 62-31947 describes an annealing treatment of the hydrogen-absorbing alloys within the temperature range of 950.degree. C. to 1250.degree. C.
To complete the electrode, the hydrogen-absorbing alloys are mechanically pulverized into granules having an average granule size of approximately 50 .mu.m or less. The granules are then kneaded into a paste with powdered polytetrafluorethylene (PTFE), which acts as a binder and a conductive agent. The paste is coated onto a conductive substrate, such as a current collector, comprising a punched metal plate.
Despite the use of various types of hydrogen-absorbing alloys or manufacturing methods used to prepare the alloys, it is difficult to improve the initial discharge characteristics of batteries employing hydrogen-absorbing electrodes. The laid-open Japanese patents, Nos.3-219036 and 3-280357, describe the use of a hydrogen-absorbing alloy as a negative electrode in an alkaline storage battery in which the alloy's main phase does not include boron (B), while a sub-phase includes boron (B). This method is advantageous in that the addition of boron (B) to the alloy, generates cracks at the first charge-discharge cycle of the battery. The area formed by the cracks easily contacted with the alkaline electrolyte in the battery. As a result of contacting the electrolyte, the charge-discharge characteristics of the electrode improves relative to the first charge-discharge cycle.
The discharge characteristics of alkaline batteries, utilizing a hydrogen-absorbing alloy of formula MmNi.sub.3.2 CoAl.sub.0.2 Mn.sub.0.6 B.sub.0.1 (referenced in the laid-open Japanese patent No. 3-280357) and a hydrogen-absorbing alloy of formula MmNi.sub.3.2 CoAl.sub.0.2 Mn.sub.0.6 (used as a comparative example) have been tested. FIG. 7, a graph of the discharge characteristics of the electrodes used as negative electrodes of the batteries, depicts the results of such testing. In FIG. 7, the abscissa represents the discharge capacity ratio at 2 C discharge of the batteries and the vertical line represents the battery voltage. In the alloy MmNi.sub.3.2 CoAl.sub.0.2 Mn.sub.0.6 B.sub.0.1, a subphase, with chemical formula MmNi.sub.2 CO.sub.2 B, is formed and observed. These tests demonstrate that the addition of boron (B) to the hydrogen-absorbing alloy prevents the deterioration of the discharge capacity and discharge voltage characteristics of the battery, even if the discharge current increases. In other words, adding boron (B) to the alloy improves the high rate discharge characteristics. The characteristics improve because adding boron (B) to the hydrogen-absorbing alloy generates cracks that form surfaces which easily contact with electrolytes. The contact with the electrolytes results in the improvement of electrode reactions.
Nonetheless, this method is not without problems. At times, the degree of dispersion of the boron (B) is small in manufacturing the hydrogen-absorbing alloy and the boron (B) occasionally coheres in the hydrogen-absorbing alloy. Consequently, the results obtained by adding the boron (B) prove insufficient. This inventor has encountered such inferior results.